1. Technical Field
This invention relates to the field of electronic control of automatic transmissions for motor vehicles. More particularly, it pertains to adaptive control of pressure supplied to friction elements that produce gear ratio changes when engaged and disengaged.
2. Description of the Prior Art
In the control of an automatic transmission, certain gear ratio changes require an element (such as a friction clutch, friction brake, brake band or one-way clutch) to disengage and another element to engage, often at approximately the same time. In order for the oncoming element to engage correctly, it must have enough torque capacity to complete the shift within a specified time. If its torque capacity is too high, the shift is usually unacceptable due to its harshness. If its torque capacity is too low, engine speed will flare as the gear ratio approaches neutral during the shift, also resulting in an unacceptable shift.
The torque capacity of a friction clutch, brake or brake band is a function of the element's characteristics and the magnitude of hydraulic pressure applied to the element. The magnitude of hydraulic pressure is the result of a command from a powertrain control module (PCM) to an electrohydraulic solenoid-operated valve supplied from a source of pressurized fluid. The solenoid preferably is a variable force solenoid (VFS), which controls the magnitude to the commanded pressure. The pressure command to the solenoid is controlled by the strategy resident in the PCM program. An objective of this invention is to determine, command and apply the pressure magnitude supplied to the friction element that will produce the best shift.
In an automatic transmission, hydraulic pressure is used to control the engaged and disengaged state of the on-coming and off-going friction elements, which control the elements of the gearsets that produce gear ratio changes. Variation in pressure supplied to the oncoming friction element, a function of the element's torque capacity, can cause variations in shift quality over the service life of the vehicle. The process of this invention for adapting friction element pressure is used to compensate for changes in torque capacity of the friction element.
Conventional open-loop hydraulic pressure control requires a specific friction element torque capacity to produce a gear ratio change. Friction element torque capacity is controlled by throttle valve pressure, which is calculated with reference to transmission input torque, shift inertia torque, gain and spring offsets of the clutch mathematics model, temperature compensation, and dynamic throttle adder. However, open-loop pressure control provides no compensation for hardware changes, component wear and degradation, unit-to-unit variability, such as variable force transfer functions and physical tolerance variations. Furthermore, inherent conditions in a powertrain, such as localized temperature effects, coefficients of friction, engine torque, and sensor drift, cannot be adequately determined and accommodated.
A current strategy for commanding pressure at the start of a gear ratio change requires knowing inferred engine torque, inertia torque, and a calibrated offset. Some disadvantages of this strategy include: (1) calculated inferred engine torque may change from actual torque due to production variations; torque calculations, and engine wear over time; (2) commanded pressure may not be the same as actual pressure due to variations in the variable force solenoid, and (3) the same pressure does not always produce the same torque capacity due to component wear and variations among components.
It is preferred to use instead an adaptive process that employs fuzzy logic techniques to adapt the initial pressure of the oncoming friction element for an upshift, and the off-going element for a downshift.
Adaptive controllers are designed to correct for nonlinear or time-varying changes in system parameters. In nonlinear systems the parameters change as the operating conditions change. This can cause problems for a controller which is tuned for only a specific condition. An adaptive controller can adjust to changes in system characteristics. This keeps the controller tuned throughout all the operating conditions.
Time-varying systems also pose a problem for controllers without adaptive capabilities. In this case the controller may stay tuned for a limited amount of time until the system parameters change. This can be seen in systems such as a transmission where degradation and wear start to change the system parameters after some time. An adaptive system can compensate for these changes in system parameters by changing the output of the systems controller.